Membrane processes such as hyperfiltration including ultrafiltration, and reverse osmossis operate with selective passage of the water from a solution or suspension of material in the water. The material retained by the membrane, or rejected, is consequently more concentrated near the membrane than it was at the entrance and more than the material more distant from the membrane. The less concentrated liquid and that which is most readily passed is thus separated from the membrane. This phenomenon is sometimes termed "concentration polarization" and is the subject of numerous articles and reports. Since boundary layer concentration is even more pronounced in laminar or less turbulent flow situations, prior efforts to solve the problems engendered thereby have included the use of more turbulent flow conditions but such adds to costs especially from increased power consumption.
While the boundary layer is believed to exist only in theory, what is meant herein by that term is simply the concentrated portions of the flow near the interface between the fluid and the membrane.